Over the past three decades there has been a strong focus in the field of addiction towards the development of vaccines for the prevention and treatment of tobacco smoking (nicotine dependence). Based on preclinical studies in rodents, the presumed mechanism of action is that the antibodies produced by vaccination, bind to nicotine and decrease the rate and the amount of nicotine that is able to cross the blood-brain barrier and occupy nicotinic acetylcholine receptors (nAChR) in the mesocorticolimbic reward pathway. In the present study, we propose to validate this mechanism in living human smokers. Specifically, we will image brain 22- nAChR using the nicotinic agonist [123I]5-IA-85380 ([123I]5-IA) and single photon emission computed tomography (SPECT) to determine the amount of nicotine occupancy of brain 22-nAChR after an IV nicotine challenge in smokers before and after vaccination with the NicVax vaccine. We hypothesize that after vaccination, the amount of nicotine occupying 22-nAChR will be reduced by at least 50%. If confirmed, this reduction in the amount of nicotine occupying nAChR will explain why, vaccinated individuals will be less likely to become addicted if they smoke, and current smokers will not experience the same satisfaction from smoking. PUBLIC HEALTH RELEVANCE Smoking is the leading known cause of preventable death and disease. Despite the debilitating medical, economic and social costs of cigarette smoking, people continue to smoke. The persistence of this destructive behavior is a consequence of insufficient smoking cessation treatments to assist smokers in their efforts to quit smoking. The nicotinic acetylcholine receptor (nAChR) is a likely neurochemical substrate of the addiction to cigarette smoking. These studies will define the brain mechanism for how the nicotine vaccine alters nicotine delivery to brain nAChR. As these studies lead to better smoking cessation treatments, they will decrease the incidence of smoking-related deaths and disease that plague the world today. [unreadable] [unreadable] [unreadable]